Conventional plate-shaped structures have been known in which a plurality of cells are disposed adjoiningly inside by sectioning a structure by polygonal-cylindrical or circular-cylindrical sidewalls. Patent Document 1, for example, describes these types of structures. The structure in Patent Document 1 is formed of a core layer made of a pair of sheets and sheet-like skin layers provided on the upper and lower surfaces of the core layer. A plurality of protruding portions is regularly placed on each sheet. Each protruding portion is formed so as to protrude from the flat portion of the sheet and has a cylindrical shape with the head portion being closed. The pair of sheets is layered on top of each other in such a state that the protruding portions face each other. The tops of the protruding portions of one sheet are joined to the flat portions of the other sheet, and the adjacent protruding portions on the two sheets are joined to each other. The structure in Patent Document 1 is sectioned to form a plurality of cells that are formed of protruding portions of the core layer and flat portions of the sheets. Each cell has an independent space that is completely sealed.
In addition, a honeycomb structure is known as an example of the above described structures. Patent Document 2, for example, describes a method for manufacturing a honeycomb structure from one sheet.
In accordance with the manufacturing method in Patent Document 2, a sheet material is molded so that a plurality of protruding portions having a predetermined shape is provided on the plastic sheet. After that, the sheet material is folded along a plurality of fold lines, and portions that make contact with each other are joined to each other so that a honeycomb structure is formed. Specifically, when one protruding portion is folded, one hollow prismatic columnar section having a plurality of cells disposed adjoiningly along the fold line is formed. As a result, a plurality of sections is disposed adjoiningly along the direction perpendicular to the fold line. Then, the adjacent sections are joined to each other to form a plate-shaped honeycomb structure.
The structures in Patent Documents 1 and 2 are not only used as a mere plate, but also used as a molded article having a three-dimensional shape after a bending process has been applied. The molded articles that are formed by applying such a bending process are used as members (for example, bottom wall portion) for forming a pallet box, as shown in FIG. 9, for example (see Patent Document 3).